Oil viscosity has a direct effect on engine friction which in turn affects engine torque output and idle speed. Therefore engine friction should be estimated or assumed by many parts of the engine control strategy including idle speed control and electronic throttle control. Oil viscosity also affects oil pressure, which in turn affects systems like VCT (variable camshaft timing) which rely on oil pressure to operate.
For traditional engine oils, viscosity changes dramatically as a function of temperature (i.e., a low viscosity index). New oils are being developed that have much higher viscosity index, so their viscosity changes much less with temperature.
Some engine control strategies include temperature modifiers which help compensate for changes in oil viscosity. For example, at low temperature, and higher viscosity, a greater throttle opening (higher airflow) is used to achieve a desired idle speed or engine output torque.
The inventors herein have recognized that these temperature modifiers may cause undesired operation if the engine is refilled with oil having a viscosity index which is significantly different than the manufacturer's recommendation. For example, temperature modifiers designed for manufacturer recommended high viscosity index oil will not change idle throttle openings much at low temperature. If the engine is refilled with low viscosity index oil, the idle speed at low temperatures will be lower than intended, and the engine may stall.
The inventors have solved these issues by a new control strategy which detects actual oil viscosity and/or viscosity index and controls the engine appropriately. In one aspect, a new control method comprises: cranking the engine during a start mode with an electric motor connected to a substantially constant source of electrical power; inferring engine oil viscosity based at least on engine oil temperature and speed of the engine while being cranked by the electric motor during the start mode; and correcting an operating parameter of the engine based on the inferred engine oil viscosity. In a more specific example, the operating parameter of the engine comprises a throttle position of a throttle plate controlling an amount of air inducted into the engine. And, the throttle plate is commanded to a throttle position based on a desired engine idle speed, oil temperature and an assumed viscosity of the oil, and the throttle position or angle is corrected for the inferred oil viscosity. In this manner, the correct idle speed will be maintained even after a significant change in viscosity after an engine oil change. Thus, the technical result is achieved by these actions.
In another aspect of the disclosure, the inventors have provided a method which learns oil viscosity index and controls the engine appropriately. In particular, the method comprises: cranking the engine during a start mode with an electric motor connected to a substantially constant source of electrical power; inferring engine oil viscosity based at least on engine oil temperature and speed of the engine while being cranked by the electric motor during the start mode; after at least two of the inferences, learning viscosity index of the engine oil from the inferred engine oil viscosities and the temperatures; and correcting an engine operating parameter based on current temperature of the engine oil and the learned viscosity index of the engine oil.
In still another aspect of the disclosure the inventors have provided a method which is particularly applicable to hybrid vehicles. In particular, the method comprises: accelerating the engine during a start mode with an electric motor until the engine reaches a predetermined speed; inferring engine oil viscosity based at least on engine oil temperature and an amount of electrical power used for the engine to reach the predetermined speed during the start mode; and correcting an operating parameter of the engine based on the inferred oil viscosity.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.